A combustion exhaust gas from a thermal electric power plant and a waste combustion plant is constituted mainly by water, a sulfur oxide (such as sulfur dioxide and sulfur trioxide), hydrogen chloride, a nitrogen oxide, carbon dioxide, nitrogen, oxygen and the like. When sulfur trioxide is contained in the exhaust gas at least in 1 ppm, the dew point of the exhaust gas often reaches 100° C. or more, and sulfuric acid condensation is liable to occur. An exhaust gas from a coal-fired thermal electric power plant and an exhaust gas from a waste combustion plant (an incineration facility for municipal waste or an incineration facility for industrial waste) contain a considerable amount of hydrogen chloride, and hydrochloric acid condensation is also liable to occur.
The temperature where sulfuric acid condensation occurs (i.e., the sulfuric acid dew point) and the temperature where the hydrochloric acid condensation occurs (i.e., the hydrochloric acid dew point) may vary depending on the combustion exhaust gas composition. In general, the sulfuric acid dew point is often approximately from 100 to 150° C., whereas the hydrochloric acid dew point is often approximately from 50 to 80° C. and thus a portion subjected to sulfuric acid dew point corrosion and a portion subjected to hydrochloric acid dew point corrosion may be formed even in the exhaust gas channel in one combustion plant. Accordingly, a material that is excellent in both sulfuric acid dew point corrosion resistance and hydrochloric acid dew point corrosion resistance is necessarily applied to a metal member at a relatively low temperature in an exhaust gas channel (such as a member constituting a duct wall and a chimney of a flue, a member of a dust collector, and a member of a heat exchanger for utilizing the heat of the exhaust gas).
A Sb-bearing steel has been known as a steel that is improved in acid dew point corrosion resistance (see PTLs 1 and 2). In particular, for improving both the sulfuric acid dew point corrosion resistance and the hydrochloric acid dew point corrosion resistance, it is said that the combined addition of Sb and Cu, and furthermore Mo is effective (see PTL 2).
However, since Sb is an expensive element, it may be a factor of cost increase of a steel material, and there is a fear in the raw material procurement in the case where a large amount of Sb is consumed as a raw material of a steel material. Furthermore, the addition of Sb may deteriorate the hot workability of the steel.
A stainless steel is also known as a material excellent in acid resistance, but there are cases where corrosion is more liable to proceed than the Sb-bearing steel depending on the concentration and the temperature of the acid. A stainless steel is expensive and is not a material that is completely satisfactory against the acid dew point corrosion.
According to the investigation made by the present inventors, et al., the characteristics of both sulfuric acid corrosion resistance and hydrochloric acid corrosion resistance can be improved by strictly controlling the amount of Cr and Mo added without the addition of Sb (see PTL 3).